1. Field of the Invention
The present invention relates generally to a heater assembly for providing a uniform temperature distribution, and more particularly to a heater assembly used in semiconductor device fabrication to provide a uniform temperature distribution across a wafer surface.
2. Description of the Related Art
A conventional semiconductor device is manufactured by forming a minute electronic circuit pattern on a substrate. The circuit pattern comprises wirings connecting numerous electronic elements. In particular, a silicon wafer (i.e., a small thin circular slice of pure silicon) is produced from an ingot of crystalline silicon. An electronic circuit is then formed on a surface of the wafer through a wafer fabrication (e.g., FAB) process. The wafer is next cut into a plurality of individual chips, and each chip is combined with a lead frame. An operating test can then be performed on the chip to ensure that the semiconductor device is fully functional.
During the FAB process, a thin film is first formed on the surface of the wafer during a deposition process. The film is then patterned to form an electronic circuit capable of performing a specific function. If the thickness of the thin film is not uniform over the entire surface of the wafer, a variety of process failures may occur, resulting in improper formation of the integrated circuit pattern.
The rate at which the material comprising the thin film is deposited on the wafer depends in large part on the temperature of the wafer. Specifically, when all other deposition conditions are held constant, the thin film is formed more quickly and hence more thickly on a wafer surface at a high temperature than at a low temperature. The deposition process is carried out while the wafer is repeatedly heated and cooled and the temperature can vary across the wafer surface, especially between a peripheral portion and a central portion of the wafer. Accordingly, the thin film may be formed non-uniformly over the wafer surface resulting in various film thickness, composition, and electrical resistance across the surface of the wafer. An integrated circuit patterned on the non-uniform thin film may consequently lose its functional stability. Temperature uniformity is therefore an essential factor in manufacturing a semiconductor device that has functional stability.
The recent trend in semiconductor technology is to provide devices having higher degrees of integration. This can be accomplished, for instance, by reducing the critical dimension of the circuit patterns thereof. Accordingly, temperature uniformity of the wafer surface is becoming increasingly important during the semiconductor device manufacturing process. Tungsten silicide (WSi) in particular, which is a popular wiring material in devices having a high degree of integration, is greatly influenced by temperature during the manufacturing process. Temperature uniformity on the wafer surface is even more important, therefore, when tungsten silicide or other temperature-sensitive materials are used for the wiring material.
Chemical vapor deposition (CVD) is conventionally used to form the thin film during the semiconductor device manufacturing process. Thermal CVD is frequently for forming such thin films. In the thermal CVD process, material is deposited through heat-induced chemical reactions of reactant gases supplied to a surface of a heated wafer. Thermal CVD processes are classified into atmospheric pressure CVD (APCVD) and low pressure CVD (LPCVD) processes based on the pressure in the CVD apparatus. LPCVD is especially suitable for depositing a metal silicide having a high melting point (such as tungsten silicide (WSi)) to form a polycide that can be used as a wiring material in a highly integrated circuit device.
An LPCVD apparatus includes a susceptor for supporting and fixing a wafer on an upper surface thereof, and a heater disposed below the susceptor to provide heat to the susceptor. More specifically, heat generated by the heater radiates to the susceptor and is conducted from the susceptor to the wafer. The temperature of the wafer surface is therefore dependent on the amount of heat conducted from the susceptor, and the conducted heat from the susceptor is mainly dependent on the amount of heat radiating from the heater. In other words, the temperature of the wafer surface is mainly dependent on the amount of heat radiating from the heater.
Unfortunately, however, even though an equal amount of heat is radiated to both the peripheral and the central portions of the wafer from the heater, the surface temperature at the peripheral portion of the wafer is generally lower than at the central portion of the wafer. This is because a significant amount of heat is lost from a side surface of the peripheral portion of the wafer. A larger amount of heat is therefore conserved at the central portion of the wafer and the surface temperature of the wafer is consequently much lower at the peripheral portion than at the central portion of the wafer.
Various attempts have been made to modify the structure of the heater assembly to reduce the temperature difference between the central and peripheral portions of the wafer. U.S. Pat. No. 6,032,211, for example, discloses a method for producing temperature uniformity at the surface of the wafer. The disclosed heating system includes a plurality of heating sections that are controlled independently to generate different amounts of heat for heating respective portions of the wafer. U.S. Pat. No. 4,981,815, entitled “METHOD FOR RAPIDLY THERMALLY PROCESSING A SEMICONDUCTOR WAFER BY IRRADIATION USING SEMICONDUCTOR OR PARABOLIC REFLECTORS”, discloses a method for making the surface temperature of the wafer uniform by applying more heat to the peripheral portion of the wafer than to the central portion of the wafer.
A widely used thermal assembly CVD apparatus (GENUS 7000) made by GENUS Co. Ltd. U.S.A includes an inner heater for heating a central portion of a susceptor and an outer heater for heating a peripheral portion of the susceptor to provide temperature uniformity on the surface of the wafer. The inner heater and outer heater are discrete (i.e., separated) from each other and are independently controlled to generate more heat at the peripheral portion than at the central portion. The increased heat emitted from the outer heater attempts to compensate for the heat loss at the side surface of the peripheral portion of the wafer. Unfortunately, however, the dual heater system does not provide temperature uniformity even when the outer heater generates more heat than the inner heater.
FIG. 1 is a schematic cross-sectional view showing a conventional dual heater system 90 of the GENUS 7000 thermal CVD apparatus. FIG. 2 is a schematic plan view of the dual heater system shown in FIG. 1. Referring to FIGS. 1 and 2, the conventional dual heater system 90 includes a susceptor 40 for supporting a wafer 50. A plurality of heaters 10 are disposed below the susceptor 40 to provide heat to the susceptor 40. An electrical power source supplies electric current to the heaters 10. A support 30 supports the heaters 10.
The heaters 10 include an outer heater 12 for heating a peripheral portion of the susceptor 40 and an inner heater 14 for heating an inner portion of the susceptor 40. The outer heater 12 and the inner heater 14 are separated from each other by a space 16 for preventing heat transfer between the outer heater 12 and the inner heater 14. In addition, the outer heater 12 and the inner heater 14 are controlled to operate independently. The inner heater 14 provides heat to most of the susceptor 40, and the outer heater 12 provides heat to the peripheral portion of the susceptor 40 along an outer circumference of the susceptor 40.
Each of the heaters 10 is made of a thin plate of graphite. Heat is generated using the internal resistance of the heaters 10 when the electric current is supplied to the heaters 10. The electrical power source includes a first source for providing current to the outer heater 12 and a second source 20 for providing current to the inner heater 14. The support 30 operates as an insulator and is not easily corroded by deposition gas or other pollutants. The support can be made of quartz, which is corrosion-resistant to acid or alkali materials except hydrogen fluoride and is very chemically stable.
When electric current is provided to the heaters 10 from the power source, heat generated from the graphite heaters 10 radiates to the susceptor 40. Heat is then conducted from the susceptor 40 to the wafer 50 disposed on top of the susceptor 40, thereby heating the wafer 50. The outer heater 12 is controlled to generate more heat than the inner heater 14.
Experiments have shown, however, that the temperature of the wafer surface varies significantly from the central portion to the peripheral portion of the wafer despite the provision of a dual heater assembly. FIG. 3 shows temperature distribution across the surface of a wafer heated by the conventional dual heater system of FIGS. 1 and 2. The results shown in FIG. 3 were obtained by measuring the temperature at 25 spots on a test (T/C) wafer during a tungsten silicide deposition process in the GENUS 7000 apparatus. During the deposition process, the internal pressure of the process chamber was maintained at 300 mtorr, and the temperatures of the outer heater and the inner heater were respectively set at 387° C. and 377° C.
As shown in FIG. 3, even though the outer heater 12 was controlled to generate more heat than the inner heater 14, the temperature of wafer surface was relatively high at the central portion of the wafer and relatively low at the peripheral portion of the wafer. The temperature difference between the highest temperature (TC13) and the lowest temperature (TC10) was 8.8° C. Accordingly, the results shown indicate that the dual heater system is insufficient to provide uniform temperature of the wafer surface.
Other experiments were also performed to find out whether or not generating more heat from the outer heater than the inner heater can reduce the temperature difference in the wafer surface. In another experiment, for example, the temperature of the outer heater is raised significantly without changing the temperature of the inner heater.
FIGS. 4A to 4D show the temperature distribution across the surface of a wafer heated by the conventional dual heater system of FIGS. 1 and 2 as the temperature of the outer heater was set to four different temperatures (375° C., 395° C., 415° C., and 450° C., respectively) while the inner heater temperature was kept constant. The temperature was measured at 25 spots on a test wafer in the GENUS 7000 apparatus having the conventional dual heater system. During each of these experiments, the temperature of the inner heater was set at 405° C.
Referring to FIGS. 4A to 4D, the results of the experiments show that even though the temperature of the inner heater was kept constant, the temperature of the central portion of the wafer increased as the temperature of the outer heater increased. In other words, the temperature difference between the central portion and the peripheral portion of the wafer is not significantly reduced through the use of separate heaters even when the amount of heat supplied by the outer heater increases. In these experiments, for example, even though the temperature of the outer heater was varied, the temperature differences between the highest and lowest temperature of the wafer surface were almost the same.
As described above, the conventional dual heater system does not provide sufficiently uniform temperature distribution across the surface of the wafer despite the ability to separately control the temperature of heaters used to heat the central and peripheral portions of the susceptor. Rather, experiments show that the temperature of the central portion of the susceptor is increased when the temperature of the peripheral portion is raised. Changing the structure and shape of the heater therefore does not guarantee temperature uniformity on the wafer surface. If the wafer surface temperature is non-uniform, the thickness of the thin film formed on the wafer surface will also be non-uniform. It would therefore be desirable to have a wafer heating system that provides a more uniform temperature across the surface of the wafer.